The present invention relates to a plasma display panel (PDP) and, more particularly, to a PDP of a three-electrode AC discharge type which is capable of operating in a stable state.
In general, a PDP has a large number of advantages of smaller thickness, lower flicker, larger contrast, larger display area, quicker response etc., and thus is expected for use as a flat panel display unit in a personal computer system or a workstation system as well as a wall television.
PDPs are categorized by the operational principle thereof into two types: a DC discharge type wherein bare electrodes are exposed to a discharge space (or discharge gas) for operation at a DC driving voltage; and an AC discharge type wherein electrodes are insulated from the discharge gas by an insulating coat for operation at an AC driving voltage. The DC discharge type is such that the discharge in the display cell continues during the period wherein the DC driving voltage is applied, whereas the AC discharge type is such that the polarities of the driving voltage are switched for maintaining the discharge. The AC discharge type PDP referred to as AC-PDP hereinafter is categorized into two types: a two-electrode type and a three-electrode type.
The structure and driving method of a conventional three-electrode AC-PDP will be described with reference to FIG. 1 showing a display cell of the conventional PDP in cross-section.
The AC-PDP includes a front substrate 11 and a rear substrate 12 opposed to each other, a plurality of electrodes disposed on the substrates 11 and 12, an array of display cells disposed at intersections of the electrodes. The electrodes includes a plurality of scanning electrodes 13 and a plurality of common electrodes 14 extending in parallel to one another, and a plurality of data electrodes 21 extending in parallel to one another and in perpendicular to the scanning electrodes 13 and the common electrodes 14.
The front substrate 11 is made of a glass plate mounting thereon the scanning electrodes 13 and the common electrodes 14 at a specified pitch. A dielectric film 15 and a protective film 16 for protecting the dielectric film 15 against the electric discharge are consecutively formed on the scanning electrodes 13 and the common electrodes 14. The rear substrate 12 is also made of a glass plate mounting thereon the data electrodes 21, on which a white dielectric film 10 and a fluorescent film 19 are consecutively formed. A plurality of ribs 17 are formed for defining a plurality of display cells and maintaining a specified gap between the glass substrates 11 and 12.
Each display cell defined by the ribs 17 functions as a discharge space, which is filled with a discharge gas including He, Ne and Xe, for example. The structure of the PDP is described, for example, in a literature xe2x80x9cSociety for Information Display ""98 Digestxe2x80x9d pp279-281, May 1998.
FIG. 2 shows a schematic top plan view of a general three-electrode AC-PDP, wherein a plurality of scanning electrodes S1, S2, . . . and a plurality of common electrodes C1, C2, . . . extend in a row direction, one of the scanning electrodes and one of he common electrodes forming an electrode pair, whereas a plurality data electrodes D1, D2, . . . extend in the column direction. A display cell or pixel 23 is formed at each intersection between the electrode pair and the data electrodes, a plurality of display cells 23 forming an array.
A separate driving scheme is generally used in current driving techniques for driving the AC-PDP, wherein a scanning period and a sustaining discharge period are separately provided. FIG. 3 shows a timing chart of driving signals used in the separate driving technique.
In FIG. 3, a first erasing pulse 31 is applied to each scanning electrode S1, S2, . . . for erasing the previous sustaining discharge in each cell, thereby effecting an initialization of all the cells. Subsequently, a preliminary discharge pulse 32 is applied to each common electrode C1, C2, . . . for conducting a preliminary discharge in each cell. The preliminary discharge functions for allowing a write discharge in each cell to start at a lower voltage.
Thereafter, a second erasing pulse 33 for erasing the preliminary discharge is applied to each scanning electrode S1, S2, . . . to control the wall charge in each cell generated on the dielectric film by the preliminary discharge. The period from the first erasing pulse to the second erasing pulse is called herein an erasing period. In the above description, although a single pulse is applied to electrodes at each of the first erasing voltage, preliminary discharge voltage and the second erasing voltage, a pulse train including a plurality of pulses may be applied in each driving voltages for achieving an even discharge in the cell area and suppressing the fluctuation of the electric load. Each driving pulse or pulse train may be applied to other electrodes other than those described above.
Subsequently, a scanning period is conducted by supplying a scanning pulse 34 consecutively to the scanning electrodes S1 to Sn for consecutive selection of the scanning electrodes S1 to Sn. In synchrony with supplying the scanning pulse 34, data pulses 35 are supplied to the data electrodes D1 to Dn depending on the display data. In each selected data electrode, to which a data pulse is supplied, a high voltage is applied for conducting a discharge between the scanning electrode 13 and the data electrode 21 to write the cell with the display data. Thus, each of the selected cells has larger positive wall charge generated by the high voltage near the scanning electrode 13 and negative wall charge generated by the high voltage near the data electrode 21. On the other hand, in each non-selected data electrode 21, to which a data pulse is not supplied, a discharge is not generated without changing the wall charge in the cell. In these procedures, a display data is stored in the display cells depending on the presence or absence of the data pulse.
After the scanning pulse 34 is supplied to all the scanning electrodes S1 to Sn, the PDP shifts into a sustaining discharge period wherein a sustaining pulse train is supplied to each electrode pair, whereby the scanning electrode and the common electrode are alternately supplied with sustaining pulses. The voltage of the pulse train is selected such that the pulse train cannot start a discharge by itself in each display cell without the wall charge generated by the write operation.
In the display cell having the larger positive wall charge, the first sustaining pulse of the pulse train having a negative polarity and supplied to the common electrode 14 applies the display cell with a voltage higher than the break-down voltage, thereby starting a sustaining discharge in association with the positive wall charge in the cell. The sustaining discharge by the first sustaining pulse stores negative wall charge near the scanning electrode 13 and positive wall charge near the common electrode 14.
A second sustaining pulse of the pulse train supplied to the scanning electrode 13 generates another sustaining discharge in association with the wall charge as generated by the first sustaining pulse, whereby wall charge having inverse polarities is stored near the scanning electrode 13 and the common electrode 14. Thereafter, similar sustaining discharges are generated by the alternate sustaining pulses. In this sustaining discharge period, the wall charge generated by the previous sustaining pulse is used for generating the next sustaining discharge in association with the next sustaining pulse. The number of sustaining discharges effected in a display cell determines the luminance or brightness of the display cell.
A combination of the erasing period, scanning period and sustaining discharge period as described above defines a sub-field of the PDP. In a gray-scale display scheme, a field for displaying one-screen image data includes a plurality of sub-fields, each sub-field generating a sustaining pulse train including an inherent number of sustaining pulses. The gray-scale display is effected by selecting an active state or an inactive state for each cell during each sub-field.
The conventional three-electrode AC-PDP as described above involves a problem in that the allowable range of the voltage for the scanning pulse 34 in the scanning period by which the PDP operates in a normal state is narrow depending on the load capacitance of the cell. Thus, the write discharge may be started in the cell which is not supplied with the data pulse, or cannot be started in the cell which is supplied with the data pulse. In addition, the sustaining discharge may be started irrespective of the presence or absence of the write discharge.
In view of the above problem involved in the conventional three-electrode AC-PDP, it is an object of the present invention to provide an improved three-electrode AC-PDP which is capable of operating in a stable state without malfunction.
The present invention provides a three-electrode AC-PDP including first and second substrates opposed to each other, a plurality of scanning electrodes and a plurality of common electrodes extending in parallel to one another on the first substrate, a first insulator film covering the scanning electrode and the common electrode, a plurality of data electrodes extending in parallel to one another and substantially in perpendicular to the scanning electrodes and the common electrodes, a second insulator film covering the common electrode, the first substrate and the second substrate defining therebetween a plurality of display cells having a discharge space, each of the display cells including a first cell portion where the data electrode opposes the scanning electrode and a second cell portion where the data electrode opposes the common electrode, at least one of the first insulator film and the second insulator film having a first capacitance per unit area in the first cell portion and a second capacitance per unit area in the second cell portion, the second capacitance per unit area being smaller than the first capacitance per unit area.
The smaller capacitance (electrostatic capacity) of the insulator film disposed in the second cell portion between the common electrode and the data electrode suppresses a discharge in the discharge space between the common electrode and the data electrode. In addition, if an undesirable discharge occurs between the common electrode and the data electrode, the sustaining discharge is suppressed because of smaller wall charge generated by the undesirable discharge due to the smaller capacitance generating a smaller electric field in the discharge space cell between the common electrode and the data electrode.
More specifically, the capacitance between the common electrode and the data electrode is defined as a serial branch of the capacitance of the first insulator film, the capacitance of the discharge space and the capacitance of the second insulator film. If the second insulator film (or first insulator film) has a smaller capacitance in the second cell portion than in the first cell portion, the discharge space in the second cell portion is subjected to a lower discharge voltage than in the first cell portion, whereby the discharge space in the second cell portion is less subjected to electric discharge. In a typical AC-PDP, the second insulator film includes a dielectric film and a fluorescent film formed on the data electrode.